Orthopedic implants are commonly used to replace some or all of a patient's joint such as a hip, knee, shoulder or elbow where deterioration of or damage to the joint has occurred due to aging, illness, injury or trauma. Orthopedic implants are commonly made of metal, ceramic, polymeric materials, or some combination thereof. For example, orthopedic implants commonly include a metal or ceramic portion that articulates against a polymeric bearing surface, or a metal surface that articulates against a facing metal or ceramic surface. In a hip replacement, the orthopedic hip implant will typically include a femoral component having a metallic or ceramic head that articulates against a polymeric (or metal or ceramic) surface of an acetabular cup. The polymeric bearing surface of the acetabular cup is typically contained within a metal or metal-containing shell which has been treated, formed, or otherwise adapted to allow for and promote bone ingrowth. The metal or metal-containing shell may incorporate, at its outer or bone contacting surface, porous titanium, tantalum, or a tantalum foam, such as Trabecular Metal™, a material used in orthopedic implants made by Zimmer, Inc., of Warsaw, Ind., and discussed in greater detail below.
In general, implants are classified as either modular or monoblock (i.e., single piece). Modular orthopedic implants are those where the implant is composed of two or more assembled parts connected by a locking mechanism and allowing for some of the constituent parts to be replaceable. For example, in a modular acetabular cup, the polymeric portion (that includes the bearing surface) is attached by mechanical means to the inside of a metal shell. With this arrangement, surgeons can replace a defective polymeric insert with another polymeric, ceramic or metal insert without disrupting the fixation of the shell to the bone. In contrast, monoblock implants are considered “single-piece” implants where the constituent components are integral and not interchangeable.
With respect to the interaction between articulating surfaces, a common concern is the generation of debris resulting from the relative motion of such surfaces. For example, motion between a metal component and a polymeric material, or the motion between facing metal surfaces may result in the release of polymeric or metallic debris into the surrounding environment. Such debris can interfere with the proper function of the orthopedic implant as well as cause an undesirable autoimmune response in the patient. For these reasons, research and development of low friction orthopedic implants that minimize the generation of debris is ongoing.
In addition to minimizing the friction between articulating surfaces and thereby limiting the production of debris, secure affixation of the implant within the bone is an extremely important aspect of joint replacement. Long-term stabilization of the implant is often achieved by incorporation or integration of the implant into the surrounding natural bone of the patient. Over time, bone from the surrounding tissue will grow into and around the implant and secure the implant to the bone tissue. This phenomenon is often referred to as osseointegration. To promote osseointegration, orthopedic implants are commonly provided with a bone-contacting surface that allows and promotes bone ingrowth. For example, many implants are provided with a porous bone-contacting surface which allows bone tissue to extend and grow into the pores of such surface. Examples of such implants are those that include a porous metal surface, such as titanium or tantalum, and implants including the previously mentioned Trabecular Metal™.
As noted above, in addition to the metal and polymeric materials used in many of the currently available implants, other materials such as ceramics have likewise been used in orthopedic implants, particularly modular implants. Ceramic materials and implants including ceramic articulating surfaces are believed to reduce the amount of debris generated by the motion of the articulating surfaces. One example of a ceramic implant is disclosed in U.S. Pat. No. 7,695,521, the contents of which are incorporated herein by reference. Another example of a ceramic-based implant is disclosed in WO 2009/103775 A2, the contents of which are also incorporated herein by reference. Ceramic materials for use with orthopedic implants and/or dental implants are described in U.S. Pat. No. 6,534,197, the contents of which are likewise incorporated herein by reference. While the use of ceramic materials in a modular implant, where the ceramic portion that includes the bearing surface is mechanically attached to a component that includes metal is known, monoblock implants (wherein the ceramic component is bonded to or otherwise attached without mechanical locking mechanisms) with an integral bone in-growth surface have been more difficult to achieve.
Accordingly, it would be desirable to provide a ceramic monoblock implant wherein the bone-contacting surface allows for a reliable and secure affixation through osseointegration with natural bone.